The invention relates to digital communications networks, and particularly, although not exclusively to an arrangement and method for transmitting multiplexed multi-user asynchronous transfer mode (ATM) traffic across such communications networks.
The known asynchronous transfer mode (ATM) transmission technique is a modem telecommunications switching technique which is able to switch connections for a wide range of different data types at a wide range of different bit rates. ATM technology provides a flexible form of transmission which allows various types of service traffic data, e.g. voice data, video data, or computer generated data to be multiplexed together onto a common physical means of transmission. Currently, several trends are encouraging the widespread introduction of ATM; for example the availability of high speed, low error rate communication links between switching centers, an availability of technology to digitize video and speech, and pressure to reduce operating costs by integrating previously separate telephony and data networks. ATM technology allows speech data, video data and inter-computer data to be carried across a single communications network. The information carried in each of these services is reduced to digitized strings of numbers which are transmitted across such a communications network from point to point.
A method of switching synchronous transfer mode cells in a circuit emulated ATM switch using a layered protocol model is described in specification No. WO-95-34977. A method of transferring ATM microcells in a telecommunications system is described in specification No. WO-96-34478.
Referring to FIG. 1 herein, there is shown schematically a portion of a communications network comprising first and second node devices 100, 101 respectively linked by a communications link 100. Transport of ATM data communications traffic is made across the communications link 102 between the first and second node devices, which may be for example switches 101, 102. Digitized data is received from customer equipment such as telephones, computers, faxes, modems and video broadcast apparatus in the form of frames of digitized signals at transmitting node, e.g. switch 100. The frames can either be of variable length or fixed length, and may arrive at the switch at a variable rate; or at a fixed rate. The frames of data arriving at the switch are packaged into ATM data cells 103, which have a fixed number of bytes. Transport of ATM cells between node devices is handled by the node devices operating in accordance with the ATM protocol corresponding to the International Standards Organization (ISO) Open Systems Inter-connexion (OSI) architecture, layers two and three(1). Packaging of the incoming data frames received asynchronously from the customer equipment is handled by the switches operating in accordance with ATM adaptation layer (AAL) protocols which segment the arriving frames of data into payload data of the ATM cells at the transmission node, and reassemble the payload data into frames at the destination node 102. The ATM adaptation layer corresponds to layer four of the OSI model. Equipment operating in accordance with the ATM adaptation layer protocols are capable of structuring incoming data in different ways, to suit different service types, e.g. video data, computer generated data, voice data. Many different service types can be implemented by the ATM adaptation layer simultaneously.
The basic reason for having ATM cells is that they have a fixed length. Fixed length cells are easier for hardware to handle than variable length frames. The ATM adaptation layer packages various types of data of variable length or fixed length frame type into the fixed length ATM cells for transport between physical devices. Because the ATM cell length was historically selected to accommodate various types of traffic, fixing the length of the ATM cell involved difficult decisions, and the final length of ATM cell selected is not perfect for each type of data carried. The ATM cell comprises a header portion which carries routing information and other housekeeping information necessary for the operation of the ATM network, and a payload portion which carries the actual data traffic. To transfer delay sensitive services such as speech, it is important that the ATM cell be reasonably short in order to avoid unacceptably long delays in filling the cell payload portion before transmitting the cell across the network. On the other hand, for other types of traffic such as computer to computer file transfers longer cells are more efficient, since the proportion of available transmission bandwidth taken up by the ATM cell header compared to the data payload of the cell is reduced. For delay insensitive traffic, the overhead of the housekeeping information sent in the header of each ATM cell would be relatively large if short cells were to be used. Thus, the choice of ATM cell size is a compromise and is settled at a length of 53 octets, comprising 48 octets of data payload (the ATM Service Data Unit, ATM-SDU) and a 5 octet header for transmission of housekeeping protocol information, as shown schematically in FIG. 2 herein. The protocol header in the ATM cell constitutes approximately 10% of the whole cell. This size of ATM cell introduces a delay in transmission of data which is significant for types of data having a low data rate, for example speech data. For example for a conventional 64 kilobits per second (kbit/s/s) voice data traffic, normal speech data samples are converted into one octet of digital data every 125 microseconds (xcexcs). Thus, 48xc3x97125 xcexcs=6000 xcexcs are required to fill the 48 data octets of an ATM cell payload. This introduces a 6 millisecond (ms) delay to each cell transmitted, in addition to two network switching delays one from each switch, and transmission delays across the network. For speech services, it is important to have an effectively constant delay between source and destination of a call, and the delay must be reasonably short. Large variations in delay produce broken sound effects, and make voice signals unintelligible to a service user. Long delays, for example those sometimes encountered on transatlantic satellite links, make two-way conversation awkward. In general, a conventionally accepted maximum one-way delay for speech data is 25 ms. Delays longer than this, as well as making the speech service unacceptable to users, also require complicated and expensive echo suppression equipment, which has the additional disadvantage of introducing noise. Thus, the conventional 53 octet ATM cell is not ideal for 64 kbit/s voice data traffic. However, with the advent of mobile telecommunications systems, normal 64 kbit/s sampled voice signals are compressed using code compression algorithms, resulting in transmission data rates as low as 4 kbit/s. Under these circumstances, the delay introduced in filling a full ATM cell may be as high as 96 ms, an unacceptably high delay.
In view of the above delays and to accommodate different data traffic types, the ATM adaptation layer (AAL) is split into a number of sub-layers. A first sub-layer, the known AAL-type 1 layer is aimed at constant bit rate services. The currently developing, and not yet finalized AAL-type 2 layer (formerly known in Europe as AAL-type 6, and elsewhere as AAL-CU) allows multiple variable length sub-cells, called mini-cells to be carried within one ATM cell. An object of AAL-type 2 is to support all services which require the multiplexing of information from multiple user data sources into a single ATM connection. The AAL-type 2 protocol, which breaks the basic rule of ATM that all cells be of fixed length, is aimed at being expedient for carrying low speed data where the delay caused by waiting for a full ATM cell to fill is too long, and the overhead of carrying an incomplete ATM cell is too great. However, the implementation of this layer is incomplete and there still remains a requirement for a method of transmitting data from a multiplicity of sources, including low bit rate data, over ATM networks in an efficient manner.
One object of the present invention is to provide an improved method for transmitting delay sensitive data from a plurality of user data sources over a communications network.
Another object of the present invention is to provide a method of transmitting multi-service data from a range of different data sources over an ATM network in an efficient manner.
Another object of the present invention is to provide a robust method of transmitting large trunk groups of data of a plurality of data user sources, over a communications network.
According to one aspect of the present invention there is provided a method of transmitting data in a communications network, said method comprising the steps of:
receiving a plurality of data samples comprising at least one said data sample received from each of a plurality of user data sources;
multiplexing said plurality of data samples into a data payload of at least one asynchronous transfer mode mini-cell;
packetizing said mini-cell(s) into at least one asynchronous transfer mode cell; and
transmitting said asynchronous transfer mode cell(s).
Preferably each said mini-cell carries data from a plurality of user data sources.
According to a second aspect of the present invention there is provided a method of communicating user data of a plurality of user data sources over a communications network, said method comprising the steps of:
multiplexing a respective data sample from each of a plurality of user data sources to produce a frame of user data, said frame containing data of each of said plurality of user data sources; and
assembling said frame into a plurality of data payloads of a corresponding plurality of asynchronous transfer mode mini-cells.
Said frame may be of a length greater than a payload of a single said mini-cell.
A said frame may be partitioned to run consecutively across a plurality of said data packet payloads.
Preferably said method comprises the step of:
assembling a respective protocol header to each of said mini-cells, said protocol header comprising a continuation indicator signal indicating whether or not said frame continues beyond a length of said mini-cell.
Preferably said continuation indicator signal comprises a single bit field.
Preferably a said mini-cell comprises an asynchronous transfer mode adaptation layer-type 2-common part sub-layer packet.
Preferably said continuation indicator signal comprises an asynchronous transfer mode adaptation layer-type 2 header.
Preferably said continuation indicator signal comprises an asynchronous transfer mode adaptation layer-type 2 service specific convergence sub-layer field.
According to a third aspect of the present invention there is provided a method of communicating user data of a plurality of user data sources over a communications network, said method comprising the steps of:
signaling an asynchronous transfer mode adaptation layer type-2 connection;
multiplexing user data of said plurality of user data sources into a trunk group frame;
assembling said trunk group frame into a payload of at least one asynchronous transfer mode variable length cell;
transmitting said asynchronous transfer mode cell(s) across said network.
According to a fourth aspect of the present invention there is provided a method of communicating user data of a plurality of user data sources over a communications network, said method comprising the steps of:
multiplexing a data sample from each of a plurality of user data sources to produce a frame of user data, said frame containing data of each of said plurality of user data sources; and
assembling said frame into a data payload of at least one asynchronous transfer mode mini-cell.
Preferably said method comprises the step of:
including a protocol header signal in a said mini-cell, to indicate a change in number of user data sources who""s data is assembled into a said at least one mini-cell.
Preferably said method comprises the step of using a packet payload type field of a service specific convergence sub-layer header of an asynchronous transfer mode mini-cell to indicate change of number of said user data sources who""s data is carried in a series of said mini-cells.
Preferably said method comprises the step of using a packet payload type field of an asynchronous transfer mode mini-cell header to signal timing of a change of a number of said multiplexed users.
Preferably said mini-cell comprises an ATM adaptation layer type-2 mini-cell.
According to a sixth aspect of the present invention there is provided a method of communicating user data to a plurality of user data sources of a communications network, said method comprising the steps of;
multiplexing data of a first plurality of user data sources into a first data group having a first group size;
assembling said first data group into a data payload of a first set of at least one mini-cell;
multiplexing data of a second plurality of user data sources into a second data group having a second group size;
assembling said second data group into a data payload of a second set of at least one mini-cells,
wherein each said mini-cell comprises a respective protocol header, said protocol header arranged to indicate a change in group size between successive mini-cells.
According to a seventh aspect of the present invention there is provided a method of transmitting data in a communications network, comprising the steps of:
receiving a plurality of data samples comprising at least one said data sample received from each of a plurality of user data sources;
multiplexing said plurality of data sources into a trunk group;
establishing a trunk group connection using an asynchronous transfer mode adaptation layer-type 2 negotiation procedure;
signaling additions or subtractions of users in the trunk group by incorporation of signals contained within an asynchronous transfer mode adaptation layer-type 2 protocol header, whilst leaving said asynchronous transfer mode adaptation layer-type 2 trunk group connection intact.
According to an eighth aspect of the present invention there is provided a method of communicating user data of a plurality of user data sources across a communications network comprising a plurality of transmitting entities and receiving entities, said method comprising the steps of:
signaling to a said receiving entity to create a trunk group connection;
multiplexing data of said plurality of user data sources into a trunk group data frame;
signaling to said receiving entity a length of said trunk group data frame;
assembling said trunk group data frame into a plurality of asynchronous transfer mode adaptation layer type-2 mini-cells; and
signaling a change in length of said trunk group data frame in an asynchronous transfer mode adaptation layer type-2 mini-cell header.
Preferably said step of signaling a length of said trunk group data frame comprises signaling by an asynchronous transfer mode negotiation procedure protocol.
Preferably said step of signaling a change in length of said trunk group data frame comprises signaling within an asynchronous transfer mode adaptation layer type-2 payload packet type field.
According to a ninth aspect of the present invention there is provided a method of implementing changes in data payload of an asynchronous transfer mode adaptation layer type-2 mini-cell by decoding a payload packet type field of said mini-cell in accordance with the following steps:
decoding a received packet payload type signal 10 as indicating a mini-cell contains a new data payload containing data from a different number of user data sources compared with a previously received mini-cell.
Preferably said method comprises the step of decoding a received packet payload type field signals as follows:
decoding a first mini-cell having packet payload type field 00 followed by an immediately succeeding second mini-cell including packet payload type signal 00 as indicating no change in a number of user data sources in a data payload of said mini-cells.
Preferably said method comprises the step of decoding a received packet payload type field signals as follows:
decoding a first mini-cell having packet payload type field 01 followed by an immediately succeeding second mini-cell including packet payload type signal 01 as indicating no change in a number of user data sources in a data pay load of said mini-cells.
Preferably said method comprises the steps of:
in an asynchronous transfer mode adaptation layer-type 2 connection, decoding a received packet payload type signal 00 of a first mini-cell and a received packet payload type signal 01 of a next received mini-cell as indicating a loss of data.
Preferably said method comprises the steps of:
in an asynchronous transfer mode adaptation layer-type 2 connection, decoding a received packet payload type signal 01 of a first mini-cell and a packet payload type signal 00 of next received mini-cell as indicating a loss of data.